Some forms of optical recording media record digital information in a material that can assume two distinct phases. Such media are often referred to as “phase-change” media. In the amorphous phase, the molecules of the material do not exhibit any long-range structure. In the crystalline phase, by contrast, the molecules possess a long-range order. The reflectivity of the material in the amorphous phase is different from the reflectivity of the material in the crystalline phase.
Phase-change material may be included in a disk as a phase-change recording layer. Digital information may be encoded in the phase-change layer by creation of regions of amorphous material and regions of crystalline material. The digital information encoded in the phase-change layer may be recovered by rotating the disk under a focused light and sensing the changes in reflectivity as the light strikes different regions of the disk.
The phase-change recording layer may be deposited on a polycarbonate substrate between dielectric layers and coated with a light reflection and heat dissipation layer. Techniques such as sputtering may be used to form the phase-change layer. The phase-change layer may be a compound comprising silver (Ag), indium (In), antimony (Sb) and tellurium (Te), although other compounds may be used as well.
When sufficiently heated, the material in the phase-change layer melts. Once melted, the material may be “quenched” or cooled into one of two phases: a crystalline phase or an amorphous phase. In general, heating the material to a high melting temperature followed by rapid cooling causes the material to assume the amorphous state. If cooling is more gradual, however, the molecules in the material have time to align themselves, and the material assumes the crystalline state. Although the material must be melted and cooled to cause it to become amorphous, the material may assume a crystalline phase at a lower temperature when heated for a longer time.
An optical recording medium typically includes a recording zone having a vast multitude of tiny regions addressable by a laser beam. The phase-change material in each region forms a data site that may be individually changed from one state to the other, thereby allowing for storage of digital data. The data sites are typically arranged in tracks called “data tracks.” Data stored on such an optical recording medium can be erased and/or written over by new data.